Apparatus and process for electrodialysis of salts

ABSTRACT

An apparatus and process produces salts by an electrodialysis operation. The basic electrodialysis apparatus is a cell having a number of compartments separated by membranes. A DC source is connected to drive a current through a feed stream passing through the cell which splits the salt stream into an acid and a base. The incoming feed may be nanofiltered to remove divalent metal. The base loop may be in communication with an ion exchange column packed with a material that removes multivalent cations. Depending upon the material being processed and the desired end result either or both the nanofiltration and the ion exchanged column may be used in the apparatus.

FIELD OF THE INVENTION

[0001] This invention relates to apparatus and processes forelectrodialysis of salts and more particularly to apparatus andprocesses that incorporate at least one of two distinct features: ananofiltration unit combined with an electrodialysis unit and an ionexchange column connected to and in communication with the base loop ofan electrodialysis cell.

[0002] This electrodialysis apparatus can be used in a number of largescale process applications. Specifically, it may be used for therecovery of lactic acid from fermentation derived ammonium lactate in atwo compartment cation cell. There may be either a nanofilter or an ionexchange column (or both) in communication with the base loop of thecation cell. The column contains a weak acid cation exchange resin.

[0003] For more information on the background of the inventivestructure, reference may be made to my co-pending applications havingthe following identifications: Process for the Recovery of Organic Acidsand Ammonia from Their Salts, Ser. No. 08/639,831, filed Apr. 92, 1996;Electrodialysis Apparatus, Ser. No. ______, filed ______; and Gasket andApparatus for Deionization, Ser. No. ______, filed ______.

[0004] The invention includes an apparatus and its related method usingan electrodialysis cell (or cells) in combination with a nanofiltrationunit for filtering an incoming monovalent salt solution in order tominimize the level of multivalent impurities. The apparatus may alsoinclude an electrodialysis cell (or cells) in combination with an ionexchange column in communication with a base loop of the saidelectrodialysis cell. The pH of the base product from the apparatus ispreferably in the range of 7 to about 13.5.

[0005] The apparatus and process are particularly well suited to theproduction of acids, especially organic acids, in conjunction with weakbases such as ammonia, or the salts of weak acids such as sodiumcarbonate or sulfite. The electrodialysis cells of the invention mayemploy membranes, such as bipolar membranes, or assemblies for splittingwater. Alternatively the splitting of water for acid, base productionmay be accomplished with a set of electrodes.

BACKGROUND OF THE INVENTION

[0006] Fermentation processes for producing organic acids, such asacetic and lactic acids, go through an intermediate production of salts,such as ammonium acetate or lactate. Hence, salts are the byproducts orintermediate products of a number of chemical processes. For example,regenerable flue gas desulfurization processes use a sodium alkali toabsorb the SO₂, thus resulting in a soluble bisulfite salt, NaHSO₃.Production of soda ash (Na₂CO₃) requires the processing of the rawmaterial salt viz., trona (Na₂CO₃.NaHCO₃.2H₂O) or the naturallyoccurring brines. In magnetohydrodynamic power generation process thepotassium carbonate seed material absorbs SO₂ in the fuel and isconverted to a byproduct potassium sulfate.

[0007] Electrodialysis (ED) may be used to convert these and othersoluble salts directly into their acid and base components. For example,such a procedure enables a direct recovery of a relatively pure form ofthe organic acid from its organic salt. The co-product base (ammonia forexample) may be recovered for reuse in the fermentation process for pHadjustment, thus permitting an economical and environmentally superioroption for producing organic acids. In other instances, such as withsodium bisulfite, trona or potassium sulfate, the electrodialysis offersan environmentally superior route for recovering or recycling the acid,base components.

[0008] Electrodialysis uses direct current as a means for causing amovement of ions in the solutions of the processing streams of saltstarting material. Electrodialysis processes are usually carried out inan arrangement comprising a stack where a plurality of flat sheet ionexchange membranes and gasket sheets are clamped together. These sheetsprovide flow paths for containing salt materials that produce acids andbases. The process unit requires a means for splitting water intohydrogen (H⁺) and hydroxyl (OH⁻) ions.

[0009] Two useful means for splitting water are:

[0010] (1) A bipolar membrane or a bipolar module formed by acombination of cation and anion membranes which functions as a bipolarmembrane. Suitable bipolar membranes are available from Aqualytics, adivision of Graver Water, and from Tokuyama Soda, and from the FormicCorporation; and

[0011] (2) An electrode set comprising an anode and a cathode. Theelectrodes, particularly the anodes, are coated for chemical stability,for minimizing power consumption, and for the formation of byproductsother than hydrogen (at cathode) and oxygen (at the anode), among otherthings. Suitable electrodes are available from the Eltech Corporation,the Electrode Products Inc., and from others. A hydrogen depolarizedanode can also generate H⁺ ions in an aqueous solution of an electrodestream next to the anode.

[0012] As described in my above-identified co-pending applications, thestack contains electrodes (anode and cathode) at either end and a seriesof membranes and gaskets which have open active areas in their middle toform a multiplicity of compartments which are separated by themembranes. Usually, a separate solution (an electrode stream) is alsosupplied to each of the compartments containing the electrodes. Specialmembranes may be placed next to the electrodes to prevent a mixing ofthe process streams with the electrode streams.

[0013] The majority of the stack between the electrode compartmentscomprises a repeating series of units of different membranes withsolution compartments between adjacent membranes. Each of the repeatingunits is called the “unit cell” or simply a “cell.” The solution issupplied to the compartments by internal manifolds formed as part of thegaskets and membranes or by a combination of internal and externalmanifolds. The stacks can include more than one type of unit cell.

[0014] Streams of processing fluids may be fed from one stack to anotherin order to optimize process efficiency. After one pass through thestack, if the change in the composition of a process stream isrelatively small, the process solutions can be recycled by being pumpedto and from recycle tanks. An addition of fresh process solution to andwithdrawal of product from the recycle loop can be made eithercontinuously or periodically in order to maintain the concentration ofproducts within a desired range.

[0015] When bipolar membranes are used to form acid or base from thesalt, in order for the membrane to function as a water splitter, thecomponent ion exchange layers must be arranged so that the anionselective layer of each membrane is closer to the anode than the cationselective layer. A direct current passed through the membranes in thisconfiguration causes water splitting with OH⁻ ions being produced on theanode side and a corresponding number of H⁺ ions being produced on thecathode side of the membranes. The dissociated salt anions move towardthe anode. The dissociated salt cations move toward the cathode.

[0016] The electrolysis process works in a similar manner, with thewater splitting occurring at the two electrodes. When a direct currentappears, water molecules are converted to oxygen gas at the anode alongwith the introduction of H⁺ ions into the aqueous solution. At thecathode, the water molecules are converted to hydrogen gas along withthe introduction of OH⁻ ions into the aqueous solution. In the hydrogendepolarized anode based electrolysis unit, OH⁻ ions are released intothe aqueous solution next to the cathode. While released, the hydrogengas is forwarded to the catalytic hydrogen depolarized anode for H⁺ iongeneration.

[0017] Electrodialysis equipment for acid/base production may have threecompartment cells comprising bipolar, cation and anion membranes; twocompartment cells containing bipolar and cation (or anion) membranes;multichamber two compartment electrodialysis cells comprising bipolarand two or more cation membranes. The term “bipolar membrane” alsoincludes bipolar equivalent structures, such as the use of electrodesand composite bipolars. FIG. 1 shows the unit cell for the three mostuseful configurations.

[0018] Specific references are:

[0019] “Electrodialysis Water Splitting Technology” by K. N. Mani; J.Membrane Sci., (1991), 58, 117-138

[0020] U.S. Pat. Nos. 4,082,835; 4,107,015; 4,592,817; 4,636,289;4,584,077; 4,390,402; and 4,536,269.

[0021] In accordance with an aspect of this invention, anelectrodialysis apparatus is improved through the addition of ananofiltration unit upstream of an electrodialysis cell or through theuse of a downstream ion exchange column in combination with theelectrodialysis cell and in communication with the base loop of thecell. Or both nanofiltration and an ion exchange column may be used.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention may be understood best by reading the followingspecification in connection with the attached drawings, in which:

[0023] FIGS. 1(a)-1(c) schematically show construction of the unit cellsfor two and three compartment electrodialysis cells using bipolarmembranes;

[0024] FIGS. 2(a), 2(b) schematically show construction of the unitcells for two and three compartment cells using a set of electrodes;

[0025]FIG. 3 schematically shows a two compartment cell using a hydrogendepolarizing electrode;

[0026] FIGS. 4(a) and 4(b) are block diagrams which show the inventionusing an electrodialysis system having an upstream nanofiltration;

[0027] FIGS. 5(a)-5(c) are block diagrams which show another apparatusof this invention comprising an electrodialysis cell in combination withan ion exchange column in communication with the base loop of theelectrodialysis cell;

[0028]FIG. 6 is a schematic diagram showing the construction of thetwo-compartment cell used to demonstrate the utility of this invention;

[0029]FIG. 7 is a block diagram showing a pilot system used for testingan apparatus of this invention;

[0030]FIG. 8 is a graph summarizing the solubility data of calcium andmagnesium, as a function of base pH;

[0031]FIG. 9 is a block diagram which shows the use of this invention inthe production of organic acid via fermentation;

[0032]FIG. 10 is a block diagram which shows another aspect of thisinvention in the production of organic acid via fermentation;

[0033] FIGS. 11(a)-11(b) are block diagrams showing flue gasdesulfurization process systems using the apparatus of this invention;

[0034]FIG. 12 is a block diagram which depicts the applicability of theinvention to the recovery of sodium carbonate from carbonate/bicarbonatecontaining mineral sources; and

[0035]FIG. 13 is a block diagram which shows a system using theinvention in conjunction with both a nanofiltered input feed and an ionexchange column at an output of said system.

CELL CONSTRUCTION

[0036]FIG. 1(a) shows a two compartment cell 20 comprising bipolar(designated as ±)membranes 22, 23 and anion (designated as −) membranes24, 26. A salt/base compartment (S/B) is located between the anionsurface of the bipolar membrane 22 and the anion membrane 24. An acidcompartment (A) is located between the cation surface of the bipolarmembrane 22 and another anion membrane 26. The combination of these twocompartments (S/B and A) and of the membranes 22-26 is termed a “unitcell” or, simply, a “cell.” Then, the cell compartments repeat, as atS/B′; and continuing on. As many as two hundred or more such cells maybe assembled between an anode (+) 28 and a cathode (−) 30.

[0037] The salt solution 32 process feedstream which is to be acidified,a lactate solution MX, for example, is fed into the salt/basecompartment S/B, while a liquid comprising water 34 may be supplied tothe acid compartment. Under a direct current driving force, the bipolarmembrane 22 splits the water, generating H⁺ and OH⁻ ions as shown (FIG.1(a)). Simultaneously, the X⁻ anions resulting from the dissociation ofthe salt stream MX are transported across the anion membrane to the acidcompartment, where they combine with the H⁺ ions to form the acid HX.The process may be represented schematically as follows:

[0038] (Salt/base compartments) MX+OH⁻−X⁻=MOH

[0039] (Acid compartments) H⁺+X⁻=HX.

[0040] The process has been detailed fully in my earlier co-pendingpatent applications. This process is best suited for processing salts ofweak bases, particularly for processing ammonium salts. Theconcentration of the acid product that can be made is in the order of1-6 N, with the higher concentrations being feasible for weak organicacids (pK_(a) of about 2.5 or greater). The feed salt concurrentlybecomes alkaline; with the pH being about 10-11 for ammonia production.

[0041]FIG. 1(b) shows a two compartment cell 36 comprising bipolar andcation membranes (designated as +). A base compartment (B) is locatedbetween the anion surface 38 of the bipolar membrane and the cationmembrane 39. A salt/acid compartment (S/A) located between the cationsurface 40 of the bipolar membrane and another cation membrane 42. Thecombination of the two membranes and the two compartments is termed a“cell.” Two hundred or more such cells may be assembled between an anodeand a cathode.

[0042] The salt solution to be acidified (an organic salt solution MX,for example) is fed to the salt/acid compartment S/A, while a liquidcomprising water may be supplied to the base compartment B. Under adirect current driving force the bipolar membrane generates H⁺ and OH⁻ions as shown in FIG. 1(b). Simultaneously the M⁺ cations resulting fromthe dissociation of the salt MX are transported across the cationmembrane to the base compartment, where they combine with the OH⁻ toform the base MOH. The process may be represented as:

[0043] (Salt/Acid compartments) MX+H⁺−M⁺=HX

[0044] (Base compartments) M⁺+OH⁻=MOH

[0045] The conversion of the salt that can be carried out efficiently bythis arrangement is determined by the amount of current used (coulombs),the concentration of the salt solution and, importantly, by the pK_(a)of the acid involved. For weakly dissociated acids with a pK_(a) greaterthan about 2.5, the conversion can be from about 80% to about 97%. Mostorganic acids such as lactic, acetic, citric, formic and other acids fitinto this category. The residual cation content in the acid product canthen be removed, if necessary, via a conventional cation exchange resin.

[0046]FIG. 1(c) shows a three compartment cell 44 using bipolar 46,cation 48 and anion 50 membranes. Three compartments, acid(A), base(B)and salt(S) are located between these three membranes, as shown. Theentire combination of membranes and compartments is termed a “cell.” Aswith the two compartment cells of FIG. 1(b), many cells may be placedbetween a single set of electrodes. This three compartment cellarrangement is the most generic for the production of acids and bases,particularly strong acids, such as hydrochloric and nitric acids, andstrong bases such as sodium hydroxide and potassium hydroxide.

[0047] The salt solution is fed into the S compartment located betweenthe cation 48 and anion 50 membranes. A liquid comprising water is fedto the acid and base compartments located on either side of the bipolarmembrane. Under a direct current driving force the H⁺ and OH⁻ ionsgenerated at the bipolar membrane 46 are transported to the acid A andbase B compartments, respectively. Concurrently, the M⁺ ions aretransported across the cation membrane 48 to the base compartment B,while the X⁻ ions are transported across the anion membrane 50 to theacid compartment A. The net effect is the production of relatively pureacid (HX) and base (MOH) products from the salt MX.

[0048] Other cell arrangements involving bipolar membranes inconjunction with two or more cation membranes or two or more anionmembranes may also be used in processing salts where the pK of theproduct acid or base is in the intermediate range. Such cellarrangements convert the salt to an acid and a base at a higher currentefficiency, as compared to the conversion of the two-compartment cellsshown in FIGS. 1(a) and 1(b), but it is also at higher capital andoperating costs.

[0049] The operation of the process using electrodes as the source of H⁺and OH⁻ ions is often termed electrolysis which involves theco-production of O₂ and H₂ at the anode and cathode, respectively.Electrolysis operation is similar to the operation of the bipolarmembrane electrodialysis described above. The main difference betweenthese two operations is the membranes which appear between an anode anda cathode. With each cell containing a set of electrodes, a number ofcells may be assembled into a single process unit. The electrical andhydraulic connections between the cells may be made in either a seriesor a parallel combination in order to form a compact commercial processunit. Exemplary references are:

[0050] Meliere, K. A., et. al., “Description and Operation of Stone &Webster/Ionics SO ₂ removal and recovery” US NTIS Report, PB-242 573,(1974), 1109-26.

[0051] U.S. Pat. No. 3,475,122.

[0052] FIGS. 2(a)-2(b) show two of the possible cell arrangements. Moreparticularly, FIG. 2(a) shows a cell 56 using two cation membranes 58,60 and three compartments located between an anode (+) and a cathode(−). The operation of the process is similar to the operation of thetwo-compartment cation cell shown in FIG. 1(b) and is particularlyapplicable to the production of weak acids from their salts. A separateacidic stream may be circulated in the compartment A′ which is a buffercompartment next to the anode 62. The salt process stream which is to beprocessed is circulated in the compartment between the two cationmembranes 58, 60. The buffer compartment A′ and cation membrane 60 areused to contain the salt stream. While the buffer compartment ispreferred, it is not essential to the operation of the two compartmentcell.

[0053] A stream comprising water is circulated in the B compartment nextto the cathode 64. Under a direct current driving force, H⁺ and OH⁻ ionsare generated at the anode and cathode, respectively, along with oxygenand hydrogen which is co-products from the dissociation of water.Simultaneously, the H⁺ ions are transported across the first cationmembrane 58 to the intermediate salt/acid (S/A) compartment where itcombines with the anion X⁻ to form the acid HX. The M⁺ cation istransported across the second cation membrane 60 to the B compartment toform the base MOH.

[0054] The reactions may be summarized as follows:

[0055] (Buffer A′ compartment) H₂O=½O₂↑+2H⁺+2e⁻

[0056] (H⁺ transported out across the first cation membrane)

[0057] (Salt/acid compartment) 2MX+2H⁺−2M⁺=2HX

[0058] (Base compartment) 2H₂O+2e⁻=2OH⁻+H₂↑2M⁺+2OH⁻=2MOH

[0059] (Overall) 2MX+3H₂O=2HX+2MOH+H₂↑+½O₂↑

[0060]FIG. 2(b) shows another version of a four compartment cell 66using two cation membranes 68, 70 and one anion membrane 72 between ananode 74 and a cathode 76. The operation of this cell 66 is similar tothe operation of the three compartment cell shown in FIG. 1(c). The cell66 is capable of generating relatively pure acid and base. The salt MXis fed to the compartment S between the anion membrane 72 and the secondcation membrane 70. The anion membrane separates the acid product fromthe feed salt. Otherwise, the operation of the cell is similar to theoperation of the two-compartment cell shown in FIG. 2(a).

[0061] When cells 56, 66 of FIGS. 2a, 2 b are compared with the bipolarmembrane based cells 20, 36, 44 (FIGS. 1(a)-1(c)), the co-production ofhydrogen and oxygen at the electrodes along with the acid and baseproducts requires an additional energy input to the process of about1.2V/cell. One option that can reduce this power load is the use of ahydrogen depolarized anode in place of a conventional anode.

[0062]FIG. 3 shows the construction of a cell 78 with a hydrogendepolarized anode, which is conceptually identical to cell 56 of FIG.2(a). In such a cell, the hydrogen gas produced at the cathode 80 isreturned to the anode 82 where it is oxidized to protons at a gasdiffusion electrode. The H⁺ ions are released into the aqueous solutionnext to the gas diffusion electrode 82. This technique can lower thecell voltage by about 1 volt/cell, thus reducing the power consumptionlevel to be somewhat nearer to that obtained with a cell using a bipolarmembrane. (Membrane & Separation Technology News, (1996), 15(2), 2-4).Other cell configurations employing the gas diffusion anodes can bevisualized by those skilled in the art.

[0063] For purposes of this disclosure, the cells employ bipolarmembranes, a combination of cation and anion membranes that behavetogether as a bipolar membrane. The cells may also employ bipolarmembrane equivalents, such as structures using electrodes and compositebipolars. The cells that have a set of electrodes generating H⁺ and OH⁻ions, a hydrogen depolarized anode based cell that collects the hydrogengas at the cathode and injects it into the companion porous catalyticelectrode to generate H⁺, may be considered equivalent. The term“bipolar membrane” or its equivalent will be used herein to denote anyone of these options.

[0064] Despite the usual filtration/ultrafiltration and carbon treatmentsteps, a major problem in using the electrodialysis cells in the watersplitting applications, is that the feed salt contains a significantamount of divalent metal ions, particularly calcium and magnesium. Whenthe feed stream is processed, the metal ions are transported to the baseloop of the electrodialysis unit. Due to their poor solubility, themetal ions are precipitated in the base loop. The precipitation of theseions inside the base loop plugs the cells, and damages the membranesthereby decreasing the current throughput and, in the extreme, causing amechanical failure resulting from an overheating and, perhaps, ameltdown.

[0065] A resin based ion exchange is a standard technique used to reducethe calcium and magnesium levels in the feed stream to the low levelsrequired for the proper operation of the electrodialysis cells. Aproblem with this approach is that the pH of the feed stream has to beraised to >9 through an addition of a base material. The feed stream isfiltered one more time (to remove any precipitates formed) in order forthe ion exchange step to be effective. Such a step is practiced, forexample, in the purification of NaCl streams in the production ofcaustic soda via electrolysis.

[0066] Many of the salts from the commercially important processes areacid or neutral. These salts may result from a fermentation of dextroseto organic acids—e.g. lactic, citric, acetic, 2-keto gulonic, and thelike. These acids have a pH range of 4 to 7 and contain significantamounts of free acids as well as calcium or magnesium which had beenadded as nutrients during the fermentation step. Such salt solutionsrequire an addition of considerable quantities of alkali in order toraise the pH to a point where an effective operation of the ion exchangecolumn can be assured. The added base would then have to be recovered inthe electrodialysis unit at an added cost in terms of the capital costof the membrane area and electrical power consumption.

[0067] Another application where acidic salt is produced is in flue gasdesulfurization. In the process, the sulfur dioxide in the flue gas isabsorbed in a solution of sodium sulfite (pH of 9.5-11) resulting in anacidic salt solution of sodium bisulfite (pH 4-5.5). The bisulfitestream can then be processed in the electrodialysis units to recover theSO₂ product and the alkali which may be recycled to the absorber. (U.S.Pat. Nos. 3,475,122; 4,082,835). However, a problem is that the fluegas, which is derived from the combustion of fossil fuels, containsflyash derived impurities, usually including calcium and magnesiumcompounds and, possibly, corrosion products (iron) from the duct workthrough which the flue gas passes. The presence of these impurities makethe processing of the bisulfite stream in the water splitter ratherdifficult, if not impossible.

[0068] A similar situation exists in the processing of impure alkalinesodium brine streams used to make sodium carbonate or sodium hydroxide.The brine stream may be from certain surface sources (such as SearlesLake in California) or from a mineral source such as trona (sodiumsesquicarbonate), derived via mining at Green River, Wyo. In a watersplitting process the mineral is acidified to liberate carbon dioxide inthe acid loop. Depending on the process choice, the acidified productmay be reacted with an additional sodium mineral either in an aboveground reactor or in an under ground mine (solution mining) to liberateCO₂ and a neutral sodium salt (e.g. sodium sulfate).

[0069] Concurrently, in the base loop, sodium carbonate is produced byreacting the caustic soda product with a portion of the bicarbonate feedor by absorbing the carbon dioxide in the sodium hydroxide generated inthe base loop. U.S. Pat. Nos. 4,584,077, 4,592,817 and 4,636,28 containexamples of such processes. In addition to containing quantities ofsodium sulfate, sodium bicarbonate, sodium chloride, and sodiumcarbonate, the minerals also contain, among other things, some calciumand magnesium compounds which could hamper their direct processing viaelectrodialysis.

[0070] In the above described and other similar applications, it wouldbe highly desirable to have improved apparatuses and processes that cantreat an acidic or near-neutral pH salt to yield the corresponding acidand an alkali at a pH of 9-14, without the need for an upstream pHadjustment and ion-exchange.

[0071] An earlier of my co-pending patent applications, Ser. No.08/639,831 discloses a use of a nanofiltration step to reduce thecalcium and magnesium levels in solutions containing ammonium salts ofmonovalent organic acids. Subsequently, such solutions are processed ina two compartment electrodialysis cell containing bipolar and anionmembranes. This process generates an ammoniacal organic salt solution ata pH of about 10 and a concentrated solution of the organic acid. Whileeffective in reducing or eliminating the precipitate formation at thebipolar membrane surface, the co-pending application does not addresseither the production of concentrated alkaline solutions or theproduction of multivalent acids.

[0072] A need exists for superior apparatuses for producing concentratedalkaline streams, e.g., ammonia, sodium sulfite, sodium carbonate,sodium hydroxide and other materials in the pH range of 9-14 and forprocesses which produce such streams.

[0073] Another need exists for an improved process that can convert saltsolutions without the need for either pH adjustment or an upstream ionexchange resin based softening step, while achieving a reliable longterm operation of the electrodialysis cell and the production ofconcentrated alkaline solutions.

[0074] Yet, another need also exists for a process that can concurrentlygenerate relatively pure acid co-products.

[0075] Still another need also exists for a novel apparatus and processthat can directly process filtered/ultrafiltered solutions of ammoniumor alkali metal salts of organic or inorganic acids to yield aconcentrated ammonia or alkali, while yielding a relatively pure acid ora substantially acidified salt stream.

SUMMARY OF THE INVENTION

[0076] The invention provides improved apparatuses, methods, andprocesses for converting a variety of salt streams into relatively pureacids and alkaline products. The alkaline product may be almost any pH,but for many applications the pH is preferably in the range of 7 toabout 13.5. The invention grew out of a number of findings: (a) Whenprocessing salts of certain organic acids, the multivalent cationsapparently bind with the organic anion. This substantially reduces theirtransport out of the salt or the salt/acid solution. (b) With theappropriate cation membranes and when dealing with weak acids, a portionof the divalent metal cations may be retained in the feed salt loop. Thebalance of these cations are transported to the base loop withoutcausing a fouling of the cation membranes. (c) The transported divalentmetals have a low but finite solubility in the alkaline productsolution.

[0077] By devising suitable apparatuses that can attain and maintain asufficiently low concentration of the divalent metals in the base loop,the precipitation of these metals either does not occur or is not aserious problem. Maintaining low concentrations of the divalent metalsin the base loop, thereby averting their precipitation, has surprisinglybeneficial effects such as a high and steady current throughput and theelimination of shunt and stray currents related heating and meltingproblems. Long term trouble-free operation of the electrodialysis cellhardware, membranes and the process are thereby achieved.

[0078] One aspect of the invention resides in processing salts of weak,low molecular weight monovalent acids. In this, the feed solution (e.g.,ammonium lactate or acetate) is subjected to nanofiltration. Thenanofilter that is most effective has a rating in the order of about 200Daltons. Thus, the molecular weight of the acid that can be efficientlyprocessed is less than about 150 Daltons. The feed may be at almost anypH, but preferably is in the range of 4-10. The nanofiltration stepproduces a filtrate wherein the divalent metals content in the saltstream is less than approximately 25 ppm total.

[0079] The purified salt stream is then processed in a two compartmentcell containing bipolar membranes (or their equivalent) and cationmembranes. The feed salt stream is fed to the salt/acid compartmentcontained between the cation side of the bipolar membrane and the cationselective membrane while a stream of water is fed into the basecompartment between the cation membrane and the anion side of thebipolar membrane. Under a direct current driving force, the feed salt isacidified in the cell to the extent that is technically and economicallyfeasible as a result of the H⁺ ions generated by the bipolar membrane.

[0080] Concurrently, the salt cation is transported to the base loopwhere it combines with the OH⁻ ions generated by the bipolar membrane toform the base product. The extent of the conversion of the salt to anacid in the acid loop is determined primarily by the amount of current(coulombs) passed through and by the pK_(a) of the acid. For weak acidshaving pK_(a)'s greater than ˜2.5, there is a conversion of 80-97%. Manyorganic acids fit this category.

[0081] This invention can be used for base products in the pH range of7-13.5, and more preferably in the pH range of 8-11. When the feedstream contains an additional salt of a stronger acid with the samecation (e.g. ammonium lactate and ammonium sulfate), the additional saltbecomes a supporting electrolyte. The formal conversion of the weak acidcan be in the order of 100%. Some times, other parts of the system maycontain weak acid which was not part of this original feed, in whichcase the weak acid conversion may be greater than 100% of the weak acidin the feed. The product acid then may have some excess H⁺ ions. Theproduct base is typically at a strength of 1-5 N.

[0082] A second aspect of the invention is subjecting the salt of themonovalent acid (molecular weight less than approximately 150) tonanofiltration, followed by processing in a three compartment cell whichcontains a bipolar membrane (or its equivalent), cation membranes andanion membranes. After nanofiltration in order to reduce its multivalentcation content to below about 25 ppm, the feed salt solution may be atany pH (preferably in the range of 4-10). The solution is fed into asalt compartment contained between cation and anion membranes. Liquidscontaining water are fed to the acid and base compartments. The acidcompartment is contained between the cation side of the bipolar membraneand the anion membrane. The base compartment is contained between theanion side of the bipolar membrane and the cation membrane. The feedsolution is depleted of its salt content in the process, while arelatively pure product acid and base at concentrations at 1-5 Nstrength are generated.

[0083] The invention can be used to generate base products in the pHrange of 7-13.5, and preferably in the pH range of 8-11. In contrastwith the two-compartment version, the three-compartment cell can be usedfor producing strong or weak low molecular weight monovalent acids.

[0084] Another aspect of the invention is a novel apparatus and processthat incorporates an ion exchange column in communication with the baseloop of the electrodialysis cell. The feed salt solution, which may beat any pH, but typically in the range of about 4-10, is suitablyfiltered to remove insoluble matter and then processed in anelectrodialysis cell containing bipolar membranes or their equivalent.

[0085] The base compartment of the cell, contained between the anionside of the bipolar membrane and the adjacent monopolar membrane, isconnected to an ion exchange column. The base solution circulatesthrough both the cell and the column. The ion exchange column containsan ion exchange resin capable of removing substantially all of themultivalent metal ions that may enter the loop, either across the cationmembrane or from the aqueous feed solution to the loop. The base productmay be at any pH in the range (5-14), particularly when the base loop iscontained between a cation membrane and the anion side of the bipolarmembrane. For an efficient removal of the multivalent cations,(particularly calcium and magnesium), the pH in the base loop ispreferably in the range of about 7-14; and most preferably about 8-11.

[0086] The electrodialysis cell may be of the two-compartment type,comprising bipolar membranes and either cation or anion membranes, or athree compartment cell containing bipolar, cation and anion membranes.Multichamber cells containing two or more monopolar membranes of thesame type as detailed in U.S. Pat. Nos. 4,536,269; 4,608,141 may alsoused as part of the improved process and apparatus.

[0087] The inventive process and apparatus has been found to beparticularly efficient when the base compartment of the electrodialysiscell is contained between the cation membrane and the anion surface ofthe bipolar membrane. This arrangement is used because additionalseparation and retention for the multivalent metals are provided by thecation membranes which facilitate the purging of these metals from theprocess stream. This reduces the amount of these metals in the baseloop, thereby saving on the cost of isolating them. Consequently the ionexchange load for these species may be substantially reduced and the ionexchange column can be made substantially smaller in relation to theamount of multivalent cations in the feed stream being processed.

[0088] This novel apparatus may be used for processing a variety ofsalts of monovalent cations, such as ammonium, sodium and potassium. Theanion of the salt may be monovalent or multivalent, (e.g., halidesorganic anions, bisulfite, sulfate, phosphate, or mixtures thereof), themain constraint being that the salts be fairly soluble in water.

DETAILED DESCRIPTION OF THE VARIOUS EMBODIMENTS

[0089] In accordance with this invention, an electrodialysis apparatusis improved through the addition of a nanofiltration unit upstream of anelectrodialysis cell or through the use of an ion exchange column incombination with the electrodialysis cell and in communication with thebase loop of the cell.

[0090] The improved apparatuses can be better understood from FIGS.4(a)-4(b), 5(a)-5(c). Other kinds of cells or cell designs can bevisualized by persons skilled in the art. FIGS. 4(a) and 4(b) show theapparatus of this invention that uses a nanofilter in conjunction withan electrodialysis units such as is shown in FIGS. 1-3.

[0091]FIG. 4(a) shows a nanofilter 90 operating upstream of atwo-compartment cation cell 92. The cell may of the type shown in orsimilar to FIGS. 1(b), 2(a), 3, or which may employ two or more cationmembranes. The nanofilter has a typical molecular weight cut off ofabout 200. Suitable filters are available from Desalination Systems,Filmtec and others.

[0092] The feed stream may be a salt of a monovalent cation andmonovalent anion, may be at almost any pH, but usually is at a pH of 4to 10. The feed stream may contain multivalent cation impurities and isinitially processed in the nanofiltration unit containing filter 90 toobtain a filtrate with a divalent metal content of about 25 ppm total.The filtered stream is fed through pipe 94 to the salt/acid compartment96 of the cell 92. When processing salts of low molecular weight (i.e.less than about 150) weak acids, this reduced level of multivalentcations has been found to be adequate for ensuring a long term,trouble-free operation of the electrodialysis cell.

[0093] The nanofiltered feed stream in the salt/acid (S/A) compartment96 of the two compartment cell 92, is usually a weak acid such aslactic, acetic, formic and the like, and is acidified by the protonsgenerated by the bipolar membrane (or its equivalent). The salt cationM⁺ is transported across the cation membrane 98 to base compartment 100.The pH of the base is produced by the input of OH⁻ ions from a bipolarmembrane in the base (B) compartment 96. The pH of the base iscontrolled to be in the range of about 7-13.5 in order to ensure atrouble-free operation of the electrodialysis cell. This pH range isnaturally achieved when a weak base, such as ammonia, is produced.

[0094] Alternatively, the pH may be kept within the target range by anaddition of a neutralizing compound, such as CO₂ sodiumbicarbonate(NaHCO₃), sodium bisulfite(NaHSO₃) or SO₂. The resultingbasic salt is a marketable product (as is the case with sodiumcarbonate) or a reusable chemical (such as sodium sulfite, Na₂SO₃ foruse in flue gas scrubbing, for example).

[0095] As opposed to the use of nanofiltration in conjunction with thetwo-compartment anion cell disclosed in my earlier co-pendingApplication (Ser. No. 08/639,831), the apparatus of FIG. 4(a) has asignificant advantage. With certain cation membranes, the monovalentcations are transported more effectively over multivalent cations, evenat the high current densities in the order of 70-100 A/ft². The cationmembranes are not readily fouled by the multivalent cations over a broadpH range. The apparatus of FIG. 4(a) takes advantage of this phenomenonto further reduce the multivalent ion concentration in the base loop ofthe electrodialysis cell, thereby ensuring that they do not precipitatein the loop.

[0096]FIG. 4(b) shows another version of the apparatus, comprising ananofilter 102 and a three compartment electrodialysis cell 104. Thesalt feed stream of the low molecular weight monovalent acid and amonovalent base is processed in the nanofilter and then supplied to thesalt loop 106 (S) of the cell. Once again, the nanofilter is able toreduce the multivalent cation content of the feed to about 25 ppm. Aswith the two-compartment cation apparatus 92 of FIG. 4(a), the cationmembrane 108 in the three compartment cell reduces the multivalentcation transport to the base loop 110, thereby further improving thelong term reliability of the process. However, in contrast with the twocompartment cation cell 92, the three compartment apparatus 104 has anextra anion membrane 112 to isolate the acid generated in the process.The three compartment apparatus 104 is capable of processing salts ofstrong or weak acids, while producing a relatively pure acid product.Feed streams containing salts of weak and strong acids can also beprocessed via this route. Once again the pH of the base product ispreferably controlled to be in the range of 7-13.5 in order to ensuretrouble-free operation of the electrodialysis cell.

[0097] FIGS. 5(a)-5(c) show another inventive apparatus that uses an ionexchange column 114 in communication with the base loop of theelectrodialysis cell. This apparatus has an advantage because it canprocess salts of multivalent acids.

[0098] The ion exchange column 114 contains a cation exchange resincapable of removing the multivalent ions, and particularly the divalentions, from the base loop solution. Since the ion exchange column can inprinciple maintain very low levels of multivalent (mostly divalent: Caand Mg) cations in the base loop, the pH of the generated base productcan cover the entire neutral gamut, i.e. pH 7-14. With a proper ionexchange column operation, dilute solutions (0-15 wt %) of a strongbase, (e.g. sodium or potassium hydroxide), can be produced. In this pHrange, a weak acid cation exchange resin is particularly desirable andeffective, but strong acid or chelating type cation resins may also beused. In one preferred mode, the pH in the base loop is maintained inthe 7-13.5 range so as to provide a certain solubility buffer for thedivalent cations. It is desirable, but not necessary, that the ionexchange resin be in the appropriate monovalent cation form prior to usein the processing operation.

[0099] Three versions of the apparatus using an ion exchange column areshown in FIGS. 5(a)-5(c). Other versions may be easily visualized bypersons skilled in the art.

[0100]FIG. 5(a) shows the ion exchange column 114 in communication withthe salt/base loop 116 of a two-compartment anion cell 118. This loop ispreferably operated in a feed and bleed mode so that the pH in the loopis maintained at the >7 level which is needed for an efficient operationof the ion exchange column. A feed stream of a salt solution is fed tothe salt/base(S/B) loop 116. The product base is withdrawn at 120, so asto achieve a requisite conversion of the feed salt. The product acid 122is withdrawn from the acid (A) loop 124.

[0101] The ion exchange column 114 maintains the multivalent cationconcentration in the base loop 116 at a level that is low enough toobtain long term trouble-free operation of the process. When the ionexchange column has been sufficiently loaded with the multivalent cationspecies, particularly Ca⁺² and Mg⁺², the column is taken out of thesalt/base recycle loop and is regenerated with acid in the conventionalmanner, and then put back into service. The apparatus of FIG. 5(a) isbest suited for processing salts of weak bases, such as ammonium nitrateor ammonium lactate.

[0102]FIG. 5(b) shows the ion exchange column 114 in conjunction with atwo-compartment cation cell 126. This configuration is useful forprocessing salts of weak acids, particularly organic acids derived fromfermentation and related processes. The feed stream supplied to thesalt/acid(S/A) loop 128 of the cell may, for example, be an ammonium orsodium salt of the organic acid at about pH of 4-7 and containsignificant quantities (e.g. about 50 ppm each) of calcium andmagnesium.

[0103] Under a direct current driving force, the salt is acidified inthe S/A loop 128, while the ammonium or sodium ions are transported tothe base loop 130. The cation membrane 132 may retain a substantialportion of the multivalent cations in the feed loop 126. However,depending on the acid being processed, the cation membrane 132 that isused, and the extent of the conversion of the feed stream to the productacid, a significant amount of the multivalent cations may gettransported across membrane 132.

[0104] In the absence of the ion exchange column, the transportedmultivalent cations will precipitate in the high pH environment of thebase loop 130, thereby preventing a reliable operation of theelectrodialysis process. However, with the ion exchange column 114 inplace, the multivalent cations are selectively and substantially removedfrom the base loop 130, thereby dramatically improving the apparatus andprocess operation. Since the ion exchange column 114 can maintain verylow levels of the divalent metals in solution, the base loop 130 may beat any neutral or alkaline pH, i.e. pH 7-14.

[0105] The one constraint is that some cation membranes 132, such as theAQ cation membranes, and the Nafion® cation membrane (DuPont) exhibitsignificant levels of calcium transport, and are somewhat easily fouledat a high pH of about 14 by the transported calcium. For this reason,one preferred pH range for the base product has been found to be a pH inthe range of about 7 to 13.5. For weak bases such as ammonia, thislimitation occurs naturally. However, when dealing with sodium andpotassium salts, mixtures thereof or mixtures of these salts withammonium salts, a suitable neutralizing compound such as SO₂, CO₂,NaHCO₃ and the like may be added to obtain a base product within thetarget pH range. An addition of a liquid comprising water may berequired in the base loop 130 in order to maintain the product baseconcentration at certain target levels. Once again the pH in the baseloop should be maintained in the >7 range in order to ensure reliableoperation of the ion exchange column 114 for removing the multivalentcations.

[0106]FIG. 5(c) shows a third version of the inventive apparatus. Here athree compartment cell 136 is used, with the ion exchange column 114once again in communication with the base loop 138; This is the mostversatile apparatus in the series of FIGS. 5(a)-5(c), since one canprocess the salts of either strong or weak acids, while yieldingrelatively pure acid and base products. Once again, in order to obtainreliable operation of the ion exchange column 114, the pH in the baseloop is maintained at pH 7 or higher. As with the two compartment cationcell 126 (FIG. 5(b)), the cation membrane 140 may retain a significantportion of the multivalent impurities in the original feed salt, therebyreducing the load on the ion exchange column 114 in the base loop 138.

[0107] Many of the commercially useful products such as ammonia, sodiumcarbonate, potassium carbonate and sodium sulfite have a pH in the rangeof 9-11. Therefore, this pH range for the base loop is the preferredrange for this apparatus. Weak acid ion exchangers have the bestperformance in terms of selectivity, capacity, stability and cost inthis pH range and, therefore, are also preferred.

[0108] The inventive apparatus and process are better understood fromthe following examples. All experiments were carried out using an eightcell, pilot, electrodialysis stack 149 that was assembled as shown inFIG. 6. All of the experiments were conducted in the two compartmentcation cell using salts of weak acids to demonstrate the apparatus andprocess.

[0109] The stack 149 included end plates 150 and 152 to which theelectrodes 154, 156 are attached and through which solutions were fedinto and removed from the stack. Gaskets used to separate the membranesand form the solution compartments A and B were 0.76 mm thick. Eachgasket had an open central area of 465 cm² (0.5 ft²), through whichcurrent could pass. The open central areas are filled with an openmeshed screen to keep the membranes separated as well as supported, andto promote good flow turbulence. Holes punched in the gaskets arealigned to form internal manifolds. Slots (ports) connecting themanifold with the open central area provide a flow of the solution intoand out of each compartment.

[0110] The stack employed a coated metal (ruthenium) oxide anode 154,supplied by Electrode Products Inc.; an electrode rinse compartment (ER)158, Sybron Chemicals MC 3475 cation membrane 160 (used because of itsadded strength) and seven repeating cells. Each cell (for example 162)includes acid compartment A 164, and a CMV, AQ or CMT cation membrane166. The AQ membrane is available from Aqualytics, a division of GraverWater, while the other membranes are products of the Asahi GlassCompany. Each cell also includes base compartment B 168 and bipolarmembrane 170, available from Aqualytics.

[0111] The last 172 of seven bipolar membrane in the stack 149 wasfollowed by an acid compartment A 174, a cation membrane (the same typeas in cells 1-7) 176, a base compartment B 178, another bipolar membrane180, an electrode rinse compartment (ER′) 182 and a stainless steelcathode 156.

[0112] The assembled stack 149 was placed in the system shownschematically in FIG. 7 in order to carry out the electrodialysisexperiments. Three pumps (P1-P3) were used to circulate solutions to theacid (190), base (192) and electrode rinse compartments from theirrespective recycle tanks 204, 202, 194 at a rate of 2-3.5 l/min. Theacid loop 196 was operated in a batch mode, while the base loop 198 wasrun in a feed and bleed mode. During operation, either fresh water or asalt solution may be added via a pump P4 from a make up tank 208, asneeded. The base and the electrode rinse tanks 202, 194 each had anominal volume of 5 liters, while the acid recycle tank 204 had thecapacity to process as much as 180 liters per batch. A cooling watercoil in the acid tank controls the temperature.

[0113] In some experiments, an ion-exchange column 206 containing a weakacid resin, IRC 84 from the Rohm & Haas Company, was used in the baserecycle loop 198. Cartridge filters 210, flow meters 212 and pressuregauges 214 were used in each loop to ensure a flow of clear fluids atknown flow rates and pressure drops in the three loops. A separate pump(not shown) was used to supply the feed salt solution to the acidrecycle tank 204. A DC power supply (not shown) was hooked up to theanode and cathode terminals 216, 218 of the stack. The requisitecontrollers for providing and controlling the electrical current inputand voltage are located in the power supply itself. Conductivity meters220 were used in the acid and base loops to monitor the progress of theelectrodialysis operation.

[0114] The system was initially charged with the requisite quantity ofthe filtered salt solution which was fed into the acid tank 204. Adilute alkaline solution along with a small amount of salt solution wasadded to the base tank 202 to provide the requisite electricalconductivity. The electrode rinse tank 194 was filled with about 5 wt %sulfuric acid. Recirculating pumps P1-P3 were started and the flows wereadjusted in order to get an inlet pressure drop of 4-9 psi in each ofthe loops. The DC current was turned on and the amperage adjusted toobtain about 40 A (80 A/ft² current density) at the start of the batch.

[0115] As the batch progressed, the conductivity of the acid solutiondecreased due to the transport of the monovalent cation (NH₄ ⁺, K⁺ orNa⁺) across the cation membranes 222, and the concurrent formation ofthe acid in the acid loop. Consequently, the cell voltage increased asthe batch progressed, until a set voltage limit of about 38V is reached(representing a unit cell voltage of about 4V, allowing 6 volts for theelectrode rinse loops). The process continued with a decreasing currentthroughput.

[0116] The process is deemed complete when a target acid conductivity,typically 10 mS/cm, is reached. In the base compartment(s) 192, themonovalent cation combines with the OH⁻ ions to form the base product.The electrical conductivity in the base loop was maintained at >10 mS/cmfor most experiments through an addition of a salt solution, if needed.The addition of CO₂ was also made to the base loop when processingsodium lactate in order to maintain the pH in the loop at <13.5.

EXAMPLES

[0117] Three different salt feeds were processed to demonstrate theusefulness of this invention. The salts, ammonium lactate, sodiumlactate and ammonium 2-keto levo gulonate (NH₄-2KLG) were products fromthe fermentation of dextrose. The pH of the salt solution ranged from4.5 to 9, the electrical conductivity of 30-60 mS/cm, and a salt contentof 70 to about 200 gm/l. All of the experiments were carried out at ornear ambient temperatures (20-32° C.)

Example 1

[0118] The pilot cell was assembled with AQ bipolar membranes and CMVcation membranes. One hundred and six liters of ultrafiltered ammoniumlactate solution was charged into the acid recycle tank 204. Theconversion to acid was monitored through conductivity and pHmeasurements. The base loop 198 did not have an ion exchange column 206.The base tank 202 was initially charged with dilute ammonium hydroxidesolution and as the ED process operated, the product ammonia solutionoverflowed from the base recycle tank 202. Small amounts of dilute NaClsolution were added to the base loop 198 to improve its conductivity.The process was deemed complete when the conductivity fell to aroundabout 7 mS/cm.

[0119] The trial lasted approximately 15 hours. Samples of acid and basewere collected and analyzed for lactic, ammonia and divalent metals. Theresults were as follows: Run Conductivi- Acid Acid Comp., Base Comp.,Time, Voltage Current ty, mS/cm pH of Volume gm/l gm/l min V A Acid BaseAcid L Lactic NH₃ NH₃ Lactic  0 0 0 42.37 23.3 5.8  106 80.4 15.3 43.7114.6  2 33.5 40 42.45 23.5 5.42  20 32 40 41.74 24.2 4.66 105.6 80.813.36 49.79 14.2  65 31.8 40 39.3 23.2 4.54 104.6 81.4 12.87 61.2 13.4131 32.3 40.1 36.2 21.3 4.36 81.7 11.9 65.33 12.2 190 32.9 40.1 33.4619.9 4.18 81.5 11.29 73.1 11.5 245 33.6 40.1 30.87 18.7 4.06 80.8 9.7159.26 11 349 35 40.1 26.51 16.7 101 355 35.1 40.1 26.3 16.5 3.84 83.68.5 81.6 10.5 480 37.2 40.1 21.1 15.1 3.63 99 83.4 6.44 84.51 10.2 48838 36.2 21.47 15.8 3.55 84.1 6.44 70.67 10.8 613 38.1 36 14.66 13.9 3.2485.2 3.89 76.5 10.6 778 38.1 31.3 9.14 13.3 2.96 96.2 87.3 2.55 76.7411.7 889 38.1 28.7 7.27 13.2 2.86 96 87   2.18 72.86 12.5

[0120] The trial produced 96 liters of product at a concentration of 87gm/l. Ammonia removal was about 87%. As can be seen, the pH in the acidloop 196 decreases as the lactate salt is converted to the acid form.The average current input to the process was calculated at 36.4 A (72.8A/ft²). Lactic loss to the base loop 198 was calculated at about 1.5%.Overall current efficiency (i.e., equivalents of ammonia transported perfaraday of current input) was approximately 55%.

[0121] The following Table summarizes the transport of the metals fromthe acid to the base loop 198 across the CMV cation membranes. Duringthe test, the Na, Ca and Mg levels in the acid decreased, while theselevels increased in the base. Also shown is the percent retention ofthese ions in the base loop 198 as a function of residual ammoniaconcentration in the acid loop. The retention figures are showncumulative, i.e., from the start of the process. Run Acid analysis, Baseanalysis, % Retention in Time, NH₃ in acid, ppm ppm acid loop min. gm/l% Conv. Na Ca Mg Fe Ca Mg Fe Na Ca Mg  0 15.3  0 164 17.2 24.8 0.58 16.136 N.D. 100  100 100  2  20 13.36 13 110 16.5 24.3 0.58 18 22.7 N.D.95.6 97.6  65 12.87 17 136 16.2 24 0.6 17.8 12.1 N.D. 93.0 95.5 131 11.924 167 15.6 23.9 0.58 14.5 9.73 N.D. 89 94.6 190 11.29 28 185 15.2 2380.57 13.6 9.84 N.D. 86.1 92.8 245 9.71 39 197 14.6 23.5 0.56 13.8 9.85N.D. ˜100  81.7 91.2 349 355 8.5 48 206 13.4 22.7 0.62 13.1 8.79 N.D.73.5 86.4 480 6.44 61 200 12 22.1 0.61 11.3 5.69 N.D. 65.5 83.6 488 6.4461 189 12 22.7 0.64 11.3 N.D. 64.5 84.4 613 3.89 77 162 11.1 22.4 0.686.8 5.65 N.D. 94 59.7 83.1 778 2.55 85 138 10.7 22.8 0.73 7.7 3.72 N.D.76 56.5 83.4 889 2.18 87 120 10.5 22.7 0.79 7.8 3.57 N.D. 66 55.2 82.9

[0122] The pH in the base loop 224 was about 10-10.5. The solubility ofthe divalent metals in the base loop 198 is estimated at 8-20 ppm forcalcium and 4-25 ppm for magnesium. As the batch progressed, thedecreasing levels of Ca and Mg in the base loop indicates a tendency forthe precipitation of these metals in the base loop with time. It can beseen that the CMV cation membranes retain significant amounts of themultivalent metals, particularly magnesium and iron. In specific termsabout 100% of the iron, about 83% of the magnesium, and about 55% of thecalcium are retained in the acid loop.

[0123] At the conclusion of the experiment, the CMV membranes werevisibly in excellent physical condition and were not fouled by thedivalent cations.

Example 2

[0124] The Example 1 was repeated after replacing the CMV cationmembranes with AQ cation membranes. A hundred six and one half liters ofthe ammonium lactate feed were processed over 817 minutes to yield aproduct acid containing 1.58 gm/l NH₃ at an average current input of33.2 A (66.4 A/ft²). Ammonia removal was about 92%. The lactic loss viadiffusion to the ammonia loop was about 2.8%. The overall currentefficiency for the process was about 68%. However, metals analysisshowed that the retention of the divalent metals was lower than for CMV;about 42% for magnesium, and about 37% for calcium. Therefore, theammonia solution from the test had higher levels of dissolved metals:about 10-19 ppm calcium and 5-31 ppm magnesium. At the end of theexperiment, the AQ cations were somewhat mottled in appearance,indicating possible fouling by the divalent cations.

Example 3

[0125] Eight batches of ammonium lactate feed containing 70-92 gm/llactate, with an initial conductivity of 28 to 42 mS/cm, were processedin the pilot cell. The cell contained AQ bipolar membranes, and CMVcation membranes that were used in Example 1. The input feed streamswere subjected to ultrafiltration (200,000 Daltons cutoff). There was noion exchange column 206 in the base loop 198. Ammonium hydroxide was ata concentration of 30-66 g/l and conductivity of 11 to 32 mS/cm wasgenerated in the base loop 198. A diffusion of a small amount of lacticanion into the base loop 198 provided the requisite conductivity in theloop. No water or salt solution addition was made during theseoperations. The batches were of varying size and lasted from 6.35 to40.3 hours. Each batch was terminated when the acid loop conductivivityhad decreased to about 7-10 mS/cm. During the batches, the ammonium ionconcentration in the acid loop 196 dropped from 7-12 gm/l to 1.1-3.6gm/l.

[0126] The total cell voltage was limited at about 38 Volts for eachbatch. The current input, which was limited at 40 A (representing aninitial current density of 80 A/ft²) decreases as the batch progressed.For each batch the average current input was calculated. The resultswere as follows: Metals in the feed, Average Values ppm Batch Duration,.Current Voltage Batch No. Ca Mg Hours A V 1 17.6 36 6.35 37 37 2 20 45 837 37 3 43 23 23.5 34 38 4 25 45 13.3 31.4 37 5 20 46 18.75 30 38 6 2446 40.3 26 38 7 ˜20 56 15 23 38 8 18 56 15.3 20.5 38

[0127] The cell was opened and inspected at the conclusion of theoperations. The bipolar membranes and CMV cation membranes were in goodcondition, with no physical evidence of fouling. However, there was acertain amount of precipitates in the base compartments 192, which waseasily washed off. The precipitate was analyzed and found be 16.4% Ca,2.5% Mg, 0.5% Na, 0.1% K and 350 ppm Fe. These operations demonstratethe progressive decrease in the current throughput, arising frompresence of the divalent metals in the feed stream and their transportto the alkaline environment in the base compartments 192. A plugging ofthe base compartments 192 and a blockage of the bipolar membrane surfaceby the divalent cations had decreased the cell performance.

Example 4

[0128] Four batch experiments were carried out using a sodium lactatefeed stream derived via fermentation. The ultrafiltered feed solutionwhich had a pH of about 5.4, contained about 105 gm/l of lactic in theform of its sodium salt as well as about 21 ppm Ca, and 62 ppm Mg. Thefeed salt had a sodium content of ˜25 gm/l. The pilot cell containedeight AQ bipolar membranes, seven CMV cation membranes (one new and sixof them from earlier Examples 1, 3,) and one new AQ cation membrane. Thecell voltage was once again limited at 38 Volts. Water was added to thebase loop 198 at the rate of 10 ml/min in order to keep the productalkali concentration below about 2.5N. There was no ion exchange column206 in the base loop 198. Carbon dioxide was bubbled into the base loopin order to maintain the pH therein below about 13.5. During theelectrodialysis process, the feed conductivity decreased from about 34mS/cm to about 9.5 mS/cm, with the residual sodium content in the acidbeing 3.5-4.0 gm/l. Details on the cell performance follow: Averagevalues Acid batch Conversion % Metals Batch Duration Voltage Currentvolume of lactate to retained in acid No: min. V A L acid % Ca Mg 1  29038 26  28→25.2 85 86 94.7 2 1218 38 26 109→98 85 81 98 3 1400 38 26132→118 82 90 93 4 1478 38 25.3 132→118 82 >99  98

[0129] The retention of the divalent metals by the cation membranes inthese operations was superior to that observed with ammonium lactate inExample 1. This is probably due to the relatively higher concentrationof the of the monovalent cation (sodium in this instance) and highercurrent efficiency for sodium vs. Ammonium (the absence of backdiffusion losses) as well as the lower conversion of the lactate salt.

[0130] The cell was opened and inspected. The bipolars and CMV cationmembranes were in excellent condition without any physical evidence offouling. The AQ cation membrane was cloudy/opaque and appeared to befouled. The internal parts of the cell were clean, because the highretention of the divalent cations by the (CMV) cation membranes resultedin low levels of divalent metals in the base loop 224 (<5 ppm Mg and <20ppm Ca). The precipitation problems will undoubtedly occur with higherlevels of the divalent metals in the feed stream, lower feedconcentration, or higher process conversions.

Example 5

[0131] A test on the conversion of ammonium-2 keto gulonic acid(NH₄-2KLG) to the free acid 2 keto gulonic acid (2KLG) was carried outin the pilot cell containing AQ bipolar and AQ cation membranes. Thestarting solution was obtained by neutralizing a fermentation derivedsample of 2KLG with ammonia, containing 170 gm/l 2KLG and 12.99 gm/l NH₃equivalents, and having a pH of about 9. Twenty eight liters of the feedwas processed in the electrodialysis cell, with the conductivitydecreasing from 35.1 mS/cm to 8.6 mS/cm due to acidification and theconcurrent transport of ammonia out of the feed loop extending to feedtank 200. The NaCl solution was added to the base loop 198 during theprocess in order to maintain a conductivity therein of 16-20 mS/cm. Onceagain, there was no ion exchange column 206 in the base loop 198. Theresults were as follows: Acid Acid loop analysis acid Base loop Run timeVoltage Current Conductivity 2KLG NH₃ Ca Mg volume analysis, ppm min V AmS/cm gm/l pH gm/l ppm ppm L Ca Mg  0 0 0 35.1 170 9.1 12.99 21.9 5.6228 24.3 0.18  6 38 34 35.4  11 35.1 40 38.2 175 8.6 11.17 20.3 4.86 270.59  21 32.7 40 35.1  27 32.3 40 38 174 4.74 10.61 19 4.52 ˜28 29.11.18  33 32.3 40 34.4  85 34.1 40 28 178 3.32 6.0 16.3 3.44 27.5 40.16.43 118 36.6 40 23.1 180 4.33 13.2 2.67 51.8 9.75 125 34.2 40 22.2 14334.9 40 19.2 181 2.73 3.35 10.4 2.04 26.5 57.1 11.5 211 38.1 40 11.5 1862.12 1.40 3.36 0.63 ˜26 79 15.8 255 38.1 38.2 9.5 189 2.01 0.84 1.280.23 25.5 87.6 17 283 38.1 37.9 8.6 190 0.73 0.59 0.13 91.7 17.7

[0132] The final product contained 190 gm/l 2KLG and only 730 ppm NH₃,representing about 95% removal of the cation from the salt. The currentefficiency was about 40%. It can be seen that substantially all of thecalcium and magnesium values in the feed salt have been transportedacross the AQ cation membranes. This is in dramatic contrast with theresults obtained with the sodium lactate test in Example 4. At least inpart, the high level of divalent cation transport is likely due to thelower retention by the AQ cation membranes (see Example 2), but may alsooccur either because 2KLG is an acid which is a much stronger acid thanlactic or 2KLG was not able to bind very well with the divalent cations.This large transport substantially increased the concentrations of 2KLGin the base loop 224. The metals, about 20 ppm for Mg and about 100 ppmfor Ca, remained in solution, since the pH in the base loop 224 was onlyin the range of 10-11.

[0133] Solubility Data for Divalent ions as a function of pH

[0134] Thirty seven batches of ammonium and sodium lactate and NH₄-2KLGfeeds were processed in the pilot assembly, with the processing of eachbatch lasting from 6 to >24 hours. The lactate feeds were from thefermentation of dextrose. The 2KLG feed was obtained by neutralizing theacid with ammonia. Each of the feeds were subjected to simple filtrationor to ultrafiltration prior to processing in the electrodialysis cell.The feeds had 20-150 ppm Ca and 6-60 ppm Mg. When processing the sodiumlactate salt, the pH of the sodium alkali base product was limited bythe addition of gaseous CO₂.

[0135] Samples of the product base were analyzed for both their divalentmetal content and their pH. There was no ion exchange column 206 in thebase loop 198, so that the measured concentrations of these ionsrepresent their solubility in the base loop. The CMV or CMT cationmembranes were used in these processings. The CMV membrane was used inthe first eighteen and the CMT membrane in the later nineteen tests.Both cation membrane remained in excellent condition after theprocessings, with no visible evidence of fouling by multivalent cationsin the feed.

[0136] The results of the study on solubility as a function of pH areplotted in FIG. 8. When producing ammoniacal base solutions the pHranged from 9 to about 11.4, while the sodium alkali solutions had a pHrange of about 12 to 13.4. The data could be divided into two sectionsfor each of calcium and magnesium. One set of data represents thesolubility limit, while the second set of data represents asupersaturated state where the alkaline solution can hold significantlyhigher levels of the divalent metals.

[0137] However, there is always the potential for spontaneousprecipitation and the consequent plugging of the base loop of theelectrodialysis cell. It should be pointed out that the base loop couldbe cleaned and the cell performance restored, as had been done onoccasion in the laboratory. The cleaning was obtained by washing theloop with a dilute solution of a strong acid, preferably HCl. However,such a step involves unscheduled downtime and reduced process throughputwith the potential for mechanical damage to cell hardware due toheating, meltdown etc. There is also a potential long term damage to thebipolar membranes as a result of heavy surface precipitation,blistering, etc.

[0138] The inventive apparatuses and processes enable a long termtrouble free operation of the electrodialysis cell by maintaining thedivalent metal concentrations in the base loop, either below or neartheir solubility limits. In the preferred pH range 9.5-11 of thisinvention, the target levels are about 2-25 ppm for Mg and about 20-100ppm for Ca. For a prolonged trouble-free operation of the ED cell, oneneeds to maintain the divalent metal at a somewhat lower level, say 2-10ppm Mg and 10-25 ppm Ca, this being governed by the dynamics of theprocess, since the anion surface of the bipolar membrane which generatesthe OH⁻ ions is at a pH of about 14. By maintaining an adequate fluidvelocity within the base compartments, there is a sustained reliablelong term operation at high current throughput.

[0139] It is important to note that the data shows a solubility of >10ppm for calcium at a pH value of 14. If such low levels of calcium canbe maintained in the base loop, the extended term production of dilutealkalis (0-15 wt %) such as sodium or potassium hydroxide can beachieved.

Example 6

[0140] Ammonium lactate made in a fermenter was filtered by using ananofiltration unit. The filter Desal 5-DK made by Desalination Systemswas used for this purpose. The product from this filtration step hadabout 90 gm/l of lactate, 10-13 gm/l ammonia as ammonium cation, 11 ppmcalcium and 9 ppm magnesium. The feed, at a pH of about 5, was thenprocessed in the pilot cell as described in Example 1. The pilot cellcontained eight AQ bipolar membranes and six CMV cation membranes takenfrom Example 1 and two new AQ cation membranes. Six consecutive batchesof about 120 liters of feed per batch were processed in a manner similarto Example 1. The results are summarized below: Average values Acidconc. Acid loop Ammonia in Batch Duration Current Voltage gm/lconductivity acid, gm/l number hr. A V Initial Final mS/cm Start End 128.1 21 39 91 94   35→8 10.44 2.7 2 24 25.7 38 76 90   38→8 13.9 2.2 325.5 26 38 82 87 39.6→8.3 15.4 2.1 4 23.7 28 38 92 98 39.1→5.6 13.9 1.145 24.7 28.5 38 93 98 39.1→7.4 11.7 2.0 6 24.5 28 38 79 90   40→6.7 Notmeasured

[0141] It can be seen that the batches were quite reproducible in termsof current input, voltage drop and conversion of the salt to acid. Atthe conclusion of the study of six batches the cell was opened. Theinternal parts were clean and free of precipitates, demonstrating thatthe use of nanofiltration, coupled with the retention of the multivalentcations afforded by the cation membranes was effective in maintainingstable long term performance.

[0142] The use of nanofiltration results in a generation of aconcentrate (termed retentate) stream that contains a portion of thefeed salt as well the bulk of the divalent metals. The stream may bedisposed of after suitable treatment. This disposal represents a lostresource. However, in many instances, such as in a fermentationoperation, the stream may be returned back into the front end andrecovered.

Example 7

[0143] A pilot system was setup in the mode shown in FIG. 7 with the ionexchange column 206 in place. The electrodialysis cell contained eightAQ bipolar membranes and eight CMT cation membranes. Both types ofmembranes were taken from the long term studies detailed earlier. Theion exchange column 206 in the base loop 198 was filled with IRC 84resin (a weak acid cation exchange resin) from Rohm and Haas andconverted to the ammonium form prior to the trials.

[0144] Feed ammonium lactate for the trials had been ultrafiltered in aunit rated at about 200,000 Daltons and contained typically 40-150 ppmCa and 45-65 ppm Mg. Lactate content in the feed ranged from 60-100gm/l.

[0145] Thirty batches of the feed ammonium lactate were processed in amanner analogous to the processing in Examples detailed before. Each ofthe batches lasted 6 to 24+ hours with each batch being terminated whenthe acid loop conductivity dropped below about 10 mS/cm. Detailedmeasurements showed the CMT and CMV membranes had similar levels ofretention for multivalent cations.

[0146] The ion exchange column was effective in maintaining the divalentmetal concentrations at low levels in the base loop. During the initialbatch following a regeneration of the column the levels of calcium andmagnesium in the base loop were in the order of 0-2 ppm each. The levelsgradually increased during subsequent batches, principally because ofthe kinetic limitations of a relatively short column (<2 feet deep) anda high service flow rate. When the divalent metal concentration reachedabout 10 ppm total, after about four batches of 140-180 liters each, thecolumn was regenerated and reused in the subsequent batches. In thismanner a stable long term operation of the electrodialysis cell wasachieved, with steady current throughputs and voltage drops.

[0147] The apparatus combining the electrodialysis cell with an ionexchange column in the base loop enabled the electrodialysis cell tooperate over extended periods without the need for routine acid cleaningof the base loop. In fact, a certain amount of buffer capacity existswithin the improved apparatus. The ion exchange column is able to cleanthe base loop simply by having the base solution circulating in theapparatus with the electrical power turned off.

[0148] For the inventive apparatuses and processes, cation membranesthat have a high level of retention for the divalent cations arepreferable because they reduce the level of these ions in the base loop.Therefore, they reduce the load on the ion exchange column. The higherretention membranes CMV and CMT have been found to be not prone tofouling by the divalent cations. It is thought that the membranes areprepared by using cross-linked polymerization of styrene and divinylbenzene onto a suitable substrate. These and similarly made cationmembranes, which we will term the “monovalent favoring type” are thepreferred ones for the inventive apparatuses of this invention. In thiscontext, the “monovalent selective” type membrane (such as the CMSmembrane from Tokuyama Soda) are the most preferable.

[0149] The apparatus of this invention can be used to improve manyprocesses involving the production of acids and bases from salts. Threesuch applications are shown schematically in FIGS. 9 through 12.

[0150]FIG. 9 shows the use of one version of the inventive apparatus inthe production of low molecular weight monovalent organic acids. A bank230 of fermenters may be operated in a batch mode to produce the organicacid in its salt form. For optimum productivity the fermentation isconducted at a pH of about 4-7. The pH is maintained through an additionof an alkali. Ammonia is a preferred alkali because of its low cost andthe ease of its recovery in a downstream electrodialysis operation.

[0151] The product organic salt is then filtered at 232 to remove anyinsoluble cell mass and subsequently nanofiltered at 234. The retentatefrom the nanofiltration unit is recycled at 236 to the fermenters. Thenanofiltrate may be further concentrated via conventional evaporation at238 if desired and fed to the acid recycle tank 246 of theelectrodialysis cells.

[0152] One or more electrodialysis process units 242, each containingtwo hundred or more electrodialysis cells may be employed to obtain therequisite product throughput. The electrodialysis (ED) cells are of thetwo-compartment cation type, such as shown in FIGS. 1(b), 2(a) or 3. Theacid loop 244 is operated in a batch mode, with the product acid beingpumped out of the acid recycle tank 246 when the target conversion isrealized. A fresh batch of feed is then added to the acid recycle tankand the process continued.

[0153] Ammonium hydroxide is generated in the base loop 248 of the EDcells. The base loop may be operated in a preferred steady state feedand bleed mode or in a batch mode. Dilution water and a small amount ofa salt solution may be added to the base loop, if necessary, in order tomaintain the product ammonia concentration and conductivity at certaintarget levels. The process is suitable for processing a number oforganic acids such as acetic and lactic.

[0154] A three compartment cell such as shown in FIG. 1(c) or FIG. 2(b)may used in place of the two compartment cation cell for producinghigher purity acids or processing salts of stronger acids.

[0155]FIG. 10 shows another version of the process employing another ofthe inventive apparatus. In this example, the product organic salt fromthe fermenters 250 is once again filtered to remove the cell mass andthe insoluble impurities via a coarse ultrafilter 252 (typically 200,000Daltons rating). The filtrate usually contains 70-110 gm/l of organicsalt. The organic salt may optionally be concentrated further viaconventional evaporation 254 prior to processing in the two compartmentED cell 256. The concentration step has the advantage that it stabilizesthe feed organic salt against further microbial growth, as well asimproving the product recovery and process efficiency of the ED recoverystep.

[0156] The ED cell 256 has an ion exchange column 258 in communicationwith the base loop 260. During the processing operation, the ionexchange column, containing a weak acid cation exchange resin, keeps themultivalent cation levels in the base loop 260 at or below theirsolubility limits (occasional excursions above the solubility may betolerated because of the built-in buffer of the ion exchange column).

[0157] Depending on the acid being produced and the product puritydesired, any one of the cells shown in FIGS. 1-3 (or similar ones) maybe used in place of the two compartment cell 256 that is shown. Theapparatus incorporating the ion exchange column 258 in communicationwith the base loop 260 of the ED cells 256 is generic and versatile. Theapparatus can process salts of either weak or strong, monovalent ormultivalent acids. Examples of acid that can be processed by theapparatus include acetic, lactic, formic, citric, gluconic and 2KLG.FIGS. 11(a)-11(b) show the applicability of the inventive apparatus inthe recovery of sulfur dioxide from flue gases. The basic process isdescribed in some of the patents cited earlier and marketed by theAlliedSignal Corporation as the SOXAL® process. In the process, sulfurdioxide from the flue gases of power plants or other sources is absorbedin a solution of sodium sulfite and sodium hydroxide (pH 9-12) to yielda salt, sodium bisulfite. In the process a certain portion of the feedsulfite is oxidized to sulfate.

[0158] In the recovery process shown in FIG. 11(a), a portion of thebisulfite product, which may have some unconverted sulfite, usually at apH of about 5-5.5, is fed to the acid compartment 264 of a twocompartment cation cell (as for example FIG. 1(b)) 266 while theremainder of the bisulfite product is fed to the base loop 268. Thebisulfite product also tends to have significant amounts of dissolvedcalcium, magnesium and other multivalent metal species derived from theflue gas source. These metals precipitate in the base loop 268 of the EDcell 266, thereby causing significant operational problems.

[0159] In the ED cell 266 of FIG. 11(a), in addition to the sodium ions,a portion of the divalent cations are transported across the cationmembranes 270 to the base loop. The divalent cations, along with thosecations added with the makeup bisulfite are removed from the base loop268 by the ion exchange column 272. The use of the inventive apparatusand process shown in FIG. 11(a) eliminates or greatly mitigates thisproblem so that long term reliable operation of the process can beachieved. The divalent cations retained in the acid loop 274 are removedalong with the sulfate after removing the SO₂ product in a stripper 276.Potassium or ammonium or mixtures of monovalent cation may be used inplace of sodium if desired.

[0160] The sodium sulfate solution from the SO₂ stripper may beprocessed in a three compartment cell after a suitable pre-treatment toremove the multivalent metals in order to generate additional alkali andbyproduct sulfuric acid. In a preferred mode the sulfate solution has acertain amount of free sulfuric acid to enable substantially a completerecovery of SO₂ in the stripper. As a result, the sulfate stream wouldbe acidic in the pH range of 3-5.

[0161]FIG. 11(b) shows the use of an improved apparatus in recoveringthe acid, base values from the acidic sulfate stream. The sulfate streamis fed to a three compartment cell 280 incorporating an ion exchangecolumn 282 in communication with the base loop 284. A portion of thesodium sulfate is converted to a byproduct sulfuric acid and a basewhich is suitable for recycling to the absorber. A portion of theunconverted sulfate values, along with the multivalent metals present init, may be discharged as a purge from the salt loop, while the balanceis recycled. As a further option, the base loops of the two and threecompartment cells in FIGS. 11(a) and 11(b) may be set in communicationwith a common ion exchange column if desired.

[0162]FIG. 12 shows the application of the inventive apparatus forprocessing impure bicarbonate/carbonate/sulfate containing streams toproduce sodium carbonate. Commercially available sodium alkali mineralsoften have impurities such as sodium sulfate, sodium chloride and acertain amount of calcium and magnesium salts. In the ED process themineral is acidified in the acid loop 286, thereby liberating carbondioxide, while sodium hydroxide is generated in the base loop 284. Thebase loop product may be acidified with a carbon dioxide containingsource to generate sodium carbonate or a similar alkaline product.

[0163] Once again the use an ion exchange column 282 in communicationwith the base loop 284 of the ED cell removes the multivalent ions,thereby assuring long term reliable operation of the overall process.Potassium sulfate streams may similarly processed to yield potassiumcarbonate.

[0164]FIG. 13 shows a process system that uses nanofiltration at 290 toremove a substantial part of the multivalent metals prior to aprocessing of the feed in the electrodialysis unit 292. The ion exchangecolumn 294 is in the base loop in order to remove any residual metalsthat might enter the base loop, thereby enhancing the reliability of theoverall process.

[0165] The combined apparatus of FIG. 13 is used with salts containingmonovalent anions, e.g., sodium, potassium or ammonium chloride,lactate, acetate et. Also, the process using both nanofiltering and anion exchange column is better suited for use in a two compartment cation(Shown in FIG. 13) cells or in three compartment cells. For twocompartment anion cells, where the feed enters the salt/base loop, andthe ion exchange column also located in the same loop, the benefit wouldappear to be less valuable.

[0166] Those who are skilled in the art will readily perceive how tomodify the invention. Therefore, the appended claims are to be construedto cover all equivalent structures which fall within the true scope andspirit of the invention.

The claimed invention is:
 1. An apparatus comprising an electrodialysiscell having at least a bipolar membrane, said bipolar membrane having acation side and an anion side, and a cation exchange membrane, saidelectrodialysis cell having at least a salt compartment, said saltcompartment being located between the cation side of the bipolarmembrane and said cation exchange membrane, means for delivering aninput stream to said salt compartment, and nanofiltration meansconnected into said means for delivering an input stream to the saltcompartment of said electrodialysis cell.
 2. The apparatus of claim 1wherein the cation membrane is selected from a group consisting of amonovalent favoring membrane and a monovalent selective membrane.
 3. Anapparatus comprising an electrodialysis cell having at least a bipolarmembrane, said bipolar membrane having a cation side and an anion side,and a cation membrane, and an anion membrane, said electrodialysis cellhaving at least a salt compartment, said salt compartment being locatedbetween the cation and the anion exchange membranes, means fordelivering an input stream to said salt compartment, and nanofiltrationmeans connected to filter said input stream while in said means fordelivering said input stream into the salt compartment of saidelectrodialysis cell.
 4. The apparatus of claim 3 wherein the cationmembrane is selected from a group consisting of a monovalent favoringmembrane and a monovalent selective membrane.
 5. An apparatus comprisingan electrodialysis cell having at least a bipolar membrane and at leastone membrane taken from a group consisting of cation and anion exchangemembranes, said bipolar membrane having a cation side and an anion side,said electrodialysis cell having at least a base loop, an ion exchangecolumn in communication with the base loop of said electrodialysis cell,said column being packed with a cation exchange resin, and means fordischarging at least part of an output stream after passing through saidbase loop.
 6. The apparatus of claim 5 wherein the cation membrane isselected from a group consisting of a monovalent favoring membrane and amonovalent selective membrane.
 7. The apparatus of claim 5 where theelectrodialysis cell comprises a bipolar membrane, a cation membrane andan anion membrane.
 8. A process for converting an incoming feed of asalt of a monovalent cation and a low molecular weight monovalent weakacid anion into an acidified product stream reduced in its monovalentcation content, said process comprising the steps of: subjecting thefeed to nanofiltration to obtain a filtrate having a total divalentmetal content which is less than about 25 parts per million; passing thefiltrate through a salt/acid compartment of a two compartmentelectrodialysis cell containing at least a bipolar membrane and a cationmembrane, said bipolar membrane having a cation side and an anion side,said salt/acid compartment being located between said cation selectiveside of the bipolar membrane and a cation membrane, the other of saidtwo compartments being a base compartment coupled in a base loop;supplying a liquid including water to the base compartment of the cell,said base compartment being located between said anion side of thebipolar membrane and a cation membrane; passing a direct current throughthe electrodialysis cell for causing an acidification of the feed saltand the concurrent transport of the monovalent cation to the base loop;producing a base product through a combination of the transported cationwith a hydroxyl ion generated by the bipolar membrane in the base loop;maintaining a pH in the base loop in the range of 7 to about 13.5; andwithdrawing the acidified feed and the base products.
 9. A process forconverting an incoming feed stream of a salt with a monovalent cationand a low molecular weight monovalent anion into an acid product streamand a base product stream, said process comprising the steps of: (a)subjecting the incoming feed stream to nanofiltration so as to obtain afiltrate having a total divalent metal content which is less than about25 parts per million; (b) passing the filtrate of step (a) through asalt compartment of a three compartment electrodialysis cell containingat least a bipolar membrane, a cation membrane, and an anion membrane,said bipolar membrane having a cation side and an anion side, said saltcompartment being located between the cation membrane and the anionmembrane, the other two compartments of said three electrodialysis cellbeing an acid cell, and a base cell coupled in a base loop; said acidcompartment being located between said cation side of the bipolarmembrane and the anion membrane and said base compartment being locatedbetween said anion selective side of the bipolar membrane and a cationmembrane; (c) supplying a liquid including water to the acid and basecompartment of the cell; (d) passing a direct current through theelectrodialysis cell for causing a conversion of at least a portion ofthe feed salt into its acid and base components; (e) maintaining a pH inthe base loop in the range of 7 to about 13.5; and (f) withdrawing thefeed depleted in its salt content, the acid and the base product.
 10. Aprocess for converting an incoming feed of a salt of a monovalent cationand a weak acid anion into an acidified product stream which is reducedin its monovalent cation content, said process comprising the steps of:(a) obtaining an input feedstream which is freed of suspended solids;(b) passing the feed of step (a) through a salt/acid compartment of atwo compartment electrodialysis cell containing at least a bipolarmembrane and two cation membranes, said bipolar membrane having a cationside and an anion side, said salt/acid compartment being located betweensaid cation side of the bipolar membrane and one of said cationmembranes, the other of said two compartments being a base compartmentcoupled in a base loop, said base compartment being located between saidanion side of the bipolar membrane and the other of said cationmembranes; (c) supplying a liquid including water to the basecompartment of the cell, said base compartment having an output streamin communication with an ion exchange column in said base loop, saidcolumn being packed with a material capable of removing multivalentcations that may enter the base loop; (d) passing a direct currentthrough the electrodialysis cell for causing an acidification of thefeed salt and a concurrent transport of monovalent cations to the baseloop; (e) producing a base product through a combination of thetransported cation with a hydroxyl ion generated by the bipolar membranein the base loop; and (f) withdrawing the acidified feed and the baseproduct.
 11. The process of claim 10 wherein the acid is an organicacid.
 12. The process of claim 10 wherein the produced base is selectedfrom a group comprising ammonia, sodium hydroxide, potassium hydroxide,potassium carbonate, sodium carbonate or mixtures thereof.
 13. A processfor converting an incoming feed of a salt of a weak base monovalentcation and an anion into a basified product stream which is reduced inits anion content, said process comprising the steps of: (a) obtaining afeed which is free of suspended solids; (b) passing the feed through asalt/base compartment of a two compartment electrodialysis cellcontaining at least a bipolar membrane and an anion membrane, saidbipolar membrane having a cation selective side and an anion selectiveside, said salt/base compartment being located between said anionselective side of the bipolar membrane and an anion membrane; saidsalt/base compartment being coupled in a base loop, the other of saidtwo compartments being an acid compartment, said salt/base compartmentbeing in communication with an ion exchange column capable of removingthe multivalent cations that may enter the acid loop; (c) supplying aliquid including water to the acid compartment of the cell, said acidcompartment being located between said cation selective side of thebipolar membrane and an anion membrane; (d) passing a direct currentthrough the electrodialysis cell for causing a basification of the feedsalt and a concurrent transport of the anion to the acid loop; (e)producing an acid product through a combination of the transported anionwith a hydrogen ion generated by the bipolar membrane in the acid loop;and (f) withdrawing the basified feed and the acid product.
 14. Theprocess of claim 13 wherein the salt which is processed is an ammoniumsalt selected from a group consisting of an organic and an inorganicacid, said acid being at least partially water soluble.
 15. The processof claim 13 where the acid which is produced is an organic or inorganicacid and the base which is produced is ammonia.
 16. The processes of anyone of the claim 8 , 9 , 10 or 13 wherein the cation membrane isselected from a group consisting of a monovalent favoring membrane and amonovalent selective membrane.
 17. A process for converting an incomingfeed of a salt of a monovalent cation and an anion into an acid productstream and a base product stream, said process comprising the steps of:(a) obtaining a feed which is free of suspended solids; (b) passing thefiltrate of step (a) through a salt compartment of a three compartmentelectrodialysis cell containing at least a bipolar membrane, a cationmembrane, and an anion membrane, said bipolar membrane having a cationselective side and an anion selective side, said salt compartment beinglocated between the cation membrane and the anion membrane, the othertwo of said three compartments being an acid compartment and a basecompartment coupled with their respective acid and base loops; (c)supplying a liquid including water to the acid and base compartment ofthe cell, said acid compartment being located between said cationselective side of the bipolar membrane and said anion membrane, saidbase compartment being located between said anion selective side of thebipolar membrane and said cation membrane, said base compartment beingin communication with an ion exchange column packed with a materialcapable of removing multivalent cations that may enter the loop; (d)passing a direct current through the electrodialysis cell for causing aconversion of at least a portion of the feed salt to its acid and basecomponents; and (e) withdrawing the feed depleted in its salt content,the acid, and the base product.
 18. The process of claim 17 where theacid is a water soluble acid selected from a group consisting ofmonoorganic, diorganic, and trivalent organic acid.
 19. The process ofclaim 17 wherein the salt which is processed is a salt selected from agroup consisting of sodium sulfite, sodium bisulfite, sodium sulfate,sodium carbonate, sodium bicarbonate, potassium carbonate, potassiumbicarbonate and mixtures thereof.
 20. The process of claim 17 wherein anacidifying agent is added into the base loop to maintain the pH in therange of 7-13.5.
 21. The process of claim 17 wherein an acidifying agentis added to the base loop to maintain the pH in the range of about 8-11within the base loop.
 22. The process of claim 17 where the salt that isprocessed is an ammonium salt.
 23. A process for the production oforganic acid, said process comprising the steps of: (a) fermentation ofa suitable substrate to generate a feed stream of a monovalent salt ofthe organic acid; (b) filtering said fermentation feed to remove solidssuspended therein; (c) supplying the filtered fermentation feed of step(b) to an acid compartment; (d) supplying a liquid including water to abase compartment, said acid compartment and said base compartmenttogether forming a two compartment cation cell apparatus, said apparatushaving an ion exchange column in communication with the basecompartment; (e) passing a direct current through the two compartmentcation cell for producing the organic acid and a base product; and (f)withdrawing said organic acid and base products.
 24. The process ofclaim 23 wherein at least some of the base product output from saidprocess is recycled to fermentation for pH adjustment.
 25. The processof claim 23 where the organic acid is lactic acid.
 26. The process ofclaim 23 wherein the base product that is produced is selected from agroup consisting of ammonia, sodium hydroxide, potassium hydroxide,sodium carbonate, potassium carbonate and mixtures thereof.
 27. Aprocess for recovering sulfur dioxide from gases, said processcomprising: (a) absorbing from a feed stream selected from a groupconsisting of a sulfite or hydroxide containing a solution of amonovalent cation for forming a mixture of bisulfite and sulfite; (b)dividing a solution derived in step (a) into two parts and feeding oneof said divided parts into a salt/acid loop and feeding the second ofsaid parts into a base loop of a two compartment cation cell; (c)transporting said part in said base loop through an ion exchange columnpacked with a material capable of removing multivalent cations that mayenter the base loop; (d) passing a direct current through said feedstream to convert the bisulfite/sulfite values to sulfur dioxide in thesalt/acid loop and concurrently producing a sulfite rich alkalinesolution in the base loop; and (e) withdrawing the said acid and baseproducts from said feed stream after it is processed through.
 28. Theprocess of claim 27 where the cation membrane is a monovalent filter ofa monovalent selective type.
 29. The process of claim 27 where themonovalent cation is selected from a group consisting of sodium,potassium and ammonium.
 30. The process of claim 27 where the baseproduct is selected from a group consisting of sodium sulfite and amixture of sodium sulfite and sodium hydroxide.
 31. The process of claim27 and the added step of stripping the acid product of its sulfurdioxide content by an application of heat, with a co-production of asulfate purge stream.
 32. The process of claim 27 and the added step ofstripping the acid product of its sulfur dioxide content by anapplication of a vacuum, with a co-production of a sulfate purge stream.33. The process of one of the claim 31 or 32 where the purge sulfatestream is further processed in a three compartment cell to produce asulfuric acid containing product and a base stream.
 34. The process ofone of the claim 26 or 32 where the base product is recycled to an SO₂absorber.
 35. A process for the production sodium alkali from impuresodium mineral sources, said process comprising the steps of: (a)filtering a sodium mineral feed solution to remove insoluble materials;(b) acidifying a portion of the feed of step (a) with a portion of anacid product from a two compartment cation electrodialysis cell, the twocompartment cell having an acid compartment and a base compartment incommunication with an ion exchange column packed with a material forremoving multivalent cations that may enter the base compartment; (c)separating carbon dioxide formed in steps (a) and (b) and forwardingsalt comprising an anion of acid to the acid compartment of theelectrodialysis cell; (d) feeding a liquid including water to the basecompartment in order to regulate the concentration of the base produced;and (e) withdrawing acid and base products produced by said process. 36.The process of claim 35 wherein the process produces sodium alkali in aform of sodium carbonate.
 37. The process of claim 35 wherein theprocess produces sodium alkali in a form of sodium hydroxide.
 38. Theprocess of claim 35 wherein the two compartment cell has a cationmembrane selected from a group consisting of a monovalent favoring typeand a monovalent selective type.
 39. A process for producing monovalentorganic acid, said process comprising the steps of: (a) fermenting asubstrate to generate a salt solution of an organic acid; (b) subjectingthe salt solution of step (a) to nanofiltration to reduce its divalentmetals content to a level below approximately 25 ppm; (c) supplying thenanofiltered salt solution of step (b) to an acid compartment of a twocompartment cation cell and supplying a liquid including water to a basecompartment of said two compartment cation cell; (d) passing a directcurrent through the two compartment cation cell for producing an organicacid and a base product; and (e) withdrawing said organic acid and baseproducts as an output of said process.
 40. The process of claim 39wherein the base product and a retentate from the nanofiltration step(b) are recycled to a fermentation of step (a).
 41. The process of claim39 wherein the two compartment cell contains a monovalent favoringcation membrane.
 42. The process of claim 39 wherein the two compartmentcell contains a monovalent selecting cation membrane.
 43. A process forproducing organic acid, said process comprising the steps of: (a)fermenting a substrate to generate a salt solution of organic acid; (b)subjecting the salt solution of step (a) to nanofiltration to reduce itsdivalent metals content to below a predetermined level; (c) supplyingthe nanofiltered salt solution of step (b) to an acid compartment of anelectrodialysis cell and supplying a liquid including water to a basecompartment of said cell, said base compartment having an ion exchangecolumn in communication with a base loop of the cell, said ion exchangecolumn capable of removing the multivalent cation entering the loop; (d)passing a direct current through the cell for producing an organic acidand a base product; and (e) withdrawing said organic acid and baseproducts as an output of said process.